Attention has been focused on a technique for forming a transistor using a semiconductor thin film formed over a substrate having an insulating surface (also referred to as a field-effect transistor (FET) or a thin film transistor (TFT)). Such a transistor is applied to a wide range of electronic devices such as an integrated circuit (IC) or an image display device (display device). A semiconductor material typified by silicon is widely known as a material for a semiconductor thin film that can be used in a transistor. As another material, an oxide semiconductor has been attracting attention. For example, a technique in which a transistor is fabricated using an In—Ga—Zn-based oxide semiconductor is disclosed (see Patent Document 1).
Furthermore, a semiconductor device achieving high field-effect mobility (simply referred to as mobility or μFE in some cases) with such a structure that a plurality of oxide semiconductor layers are stacked, the oxide semiconductor layers functioning as a channel in the plurality of oxide semiconductor layers contains indium and gallium, and the proportion of indium is higher than the proportion of gallium is disclosed (see Patent Document 1).
Non-Patent Document 1 discloses a homologous series represented by In1−xGa1+xO3(ZnO)m (−1≤x≤1, and m is a natural number). Furthermore, Non-Patent Document 1 discloses a solid solution range of a homologous series. For example, in the solid solution range of the homologous series in the case where m is 1, x ranges from −0.33 to 0.08, and in the solid solution range of the homologous series in the case where m is 2, x ranges from −0.68 to 0.32.